1. Field of the Invention
Aspects of the present invention relate to a fusing unit which heats and fuses an image transferred to a printing medium, and an image forming apparatus using the same; and more particularly, to a fusing unit which intensively heats a fusing part and enhances heat efficiency, and an image forming apparatus using the same.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus scans light to a photosensitive body which is charged to a predetermined electric potential to form an electrostatic latent image, and develops the image with a predetermined toner to transfer and fuse the image on a printing medium, thereby printing an image. To fuse the transferred image to a printing medium, the electrophotographic image forming apparatus includes a fusing unit arranged on a printing path, through which the printing medium travels.
As shown in FIG. 1, a conventional fusing unit fuses a toner image T formed on a printing medium M. The fusing unit includes a fusing roller 3 which includes a heating lamp 1 therein, a pressing roller 5 which faces the fusing roller 3 and is elastically biased by an elastic member 7 toward the fusing roller 3 to form a fusing nip N, and a temperature sensor 9.
The fusing roller 3 includes a first core pipe 3a made of a metal material, and a first elastic layer 3b which is formed on an external surface of the first core pipe 3a. Radiant energy, which is generated by the heating lamp 1, is converted into thermal energy by a light-heat conversion layer (not shown) formed in an internal surface layer of the first core pipe 3a, thereby heating the first core pipe 3a. The first elastic layer 3b is heated by heat conduction so as to provide and maintain a predetermined fusing temperature.
The temperature sensor 9 senses a surface temperature of the first elastic layer 3b. Power, which is supplied to the heating lamp 1, may be controlled based on the surface temperature sensed by the temperature sensor 9.
The pressing roller 5 includes a second core pipe 5a made of a metal material, and a second elastic layer 5b which is formed on a surface of the second core pipe 5a. The second elastic layer 5b is more elastic than the first elastic layer 3b. Thus, when the pressing roller 5 and the fusing roller 3 contact each other, the second elastic layer 5b becomes deformed.
When the printing medium M on which the toner image T is delivered to the fusing unit, the toner image T is heated and pressed while passing through the fusing nip N formed between the fusing roller 3 and the pressing roller 5 that rotate. Then, the toner image T is fused on the printing medium M to complete the fusing process.
To provide a quicker fusing of color electrophotographic images, it is necessary to enlarge an external diameter of the fusing roller 3 and the pressing roller 5 of the fusing unit or increase the thickness of the first and second elastic layers 3b and 5b resulting in an increase in the width of the fusing nip N which increases a time in which the printing medium M remains in the fusing nip N. As such, fusing quality is maintained while increasing printing speed.
However, expanding the external diameters of the fusing roller 3 and the pressing roller 5 is limited given consideration of the overall size of the image forming apparatus. Also, the expansion causes slower warm-up and raises production costs.
The expansion of the thickness of the first and second elastic layers 3b and 5b to increase the time in which the printing medium M remains in the fusing nip N also makes the warm-up slower. Further, the temperature of the first core pipe 3a necessarily increases to maintain the surface temperature of the thicker first elastic layer 3b at a fusing temperature. Thus, a junction between the first core pipe 3a and the first elastic layer 3b, and also the first elastic layer 3b, deteriorate due to the high temperature, and durability thereof is decreased.
Also, opposite end portions of the fusing roller 3 are intensively pressed when pressing the fusing roller 3 and the pressing roller 5. Thus, a center portion of the fusing roller 3 may be bent. As the fusing nip N in the center portion of the fusing roller 3 becomes smaller than that in the opposite end portions thereof, fusibility of the center portion is significantly decreased.
In the fusing unit employing the fusing roller 3, the heat generated by the heating lamp 1 is radially radiated and heats the fusing roller 3, thereby lowering heat efficiency.